Why does Iterable not provide stream() and parallelStream() methods?

Question

I am wondering why the Iterable interface does not provide the stream() and parallelStream() methods. Consider the following class:

Answer 1

If you know the size you could use java.util.Collection which provides the stream() method:

public class Hand extends AbstractCollection {
   private final List list = new ArrayList<>();
   private final int capacity;

   //...

   @Override
   public Iterator iterator() {
       return list.iterator();
   }

   @Override
   public int size() {
      return list.size();
   }
}

And then:

new Hand().stream().map(...)

I faced the same problem and was surprised that my Iterable implementation could be very easily extended to an AbstractCollection implementation by simply adding the size() method (luckily I had the size of the collection :-)

You should also consider to override Spliterator spliterator().